How do galaxies differ from each other?

in size, shape, and the ages of their stars.

Irregular galaxies are composed of young stars, while elliptical galaxies contain mostly older stars. Spiral galaxies are composed of stars of various ages, with the youngest stars in the arms.

Galaxies are classified by their shape. Each type has different characteristics and a different history of evolution. ... Elliptical galaxies have less dust than their spiral counterparts, and so the star-making process has all but ended. Most of their stars are older.

Explanation:

The most obvious difference is their shape: spiral galaxies have arms; elliptical galaxies range in shape from round to oval; irregular galaxies have no regular shape. Irregular galaxies are composed mostly of young stars, while elliptical galaxies contain mostly older stars.

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